1. Field of the Invention
The present invention relates to position detection and control system for a Direct Current (DC) motor.
2. Description of Related Art
Often position control of a DC motor requires feedback about the position of the motor shaft. Typically, a position sensor, such as an optical or Hall Effect encoder, or resolver is used to obtain the motor shaft position. The use of a position sensor increases the cost, size and weight of the system, and reduces the reliability and environmental compatibility of the system. For applications where the output speed of the motor is rather low, such as an actuator that consists of a DC motor and a reduction gear mechanism, a potentiometer is also commonly used to sense the position of the output shaft. This position sensing technique, however, is known to have poor position accuracy, is sensitive to environmental conditions such as temperatures, has poor durability due to the mechanical contact between the wiper and the resistive trace, and has high system cost due to additional wiring required between the motor and the controller.
Another known technique for obtaining motor position information is sensing the motor current directly for detecting and counting the commutation pulses as disclosed in U.S. Pat. No. 5,798,624, in which the current flowing through the lower legs of the H-bridge, same as the motor current, is monitored directly by a current sensing mechanism. The converted voltage signal of the sensed current is conditioned by using a band pass filter for extracting the commutation pulses and then fed to a pulse generator. The output of the pulse generator is then provided to a microprocessor for pulse counting to determine the motor position. Though the technique disclosed in U.S. Pat. No. 5,798,624 resolved several problems associated with the designs that use position sensors or potentiometers as mentioned above, it still suffers drawbacks. For example, it requires a special H-bridge if the sensor is located in the lower legs of the switches for capturing pulses during braking mode. Further, the system would require two sensors for bi-directional operations thereby increasing the cost of the system. If the sensor is located in the battery return, the system cannot capture commutation pulses in braking mode. In addition, the system has poor useful signal sensitivity/accuracy since the entire motor current including the main DC component is embedded in the sensor signal. The system may have pulse missing problems during start-up and stop coasting due to the use of a fixed band pass filter on the motor current signal. Further, the system may gain pulses due to brush bounces. Furthermore, since the main motor current goes through the current sensor, the system will have excessive voltage drops or power losses associated with the sensor. Also, the captured signal varies among production motors of the same design and over the life span of the same motor due to the use of current pulses associated with delayed commutation.
Yet another known technique for obtaining motor or actuator position information is sensing the motor terminal voltage directly for detecting and counting the commutation pulses as disclosed in U.S. Pat. No. 6,078,154, in which two high pass filters are used to capture the high frequency portion of the motor terminal voltage. The captured voltage signal is then fed through a low pass filter such that both DC component and high frequency noise in the sensed voltage signal are eliminated. The signal is further conditioned and fed to a pulse counter to determine motor position. This design solved the additional voltage drop and power loss problem that exists in U.S. Pat. No. 5,798,624. However, it still suffers significant drawbacks. The cost of the system is high due to the need for two current sources in the signal conditioning circuit and the need for a charge pump. Also, the system only works with MOSFET based H-bridge modules, not with bipolar transistors. The system may have pulse missing problems during motor start-up and stop due to the use of fixed-value high pass filters, and may have pulse gaining problems due to brush bounces. Furthermore, the captured signal may vary among production motors of the same design and over the life span of the same motor due to the use of current pulses associated with delayed commutation.
Still yet another known technique for obtaining motor position information is sensing the rate of change of motor current for detecting and counting the commutation pulses as disclosed in U.S. Pat. No. 6,437,533 B1. An inductor is placed at the lower side of the H-bridge to measure directly the rate of change of motor current as it flows through the lower legs of the H-bridge or through the battery return. The voltage across the inductor, L*(di/dt), is monitored, conditioned, and fed to a pulse generator circuit. The output pulse train is then provided to a microprocessor for pulse counting thereby obtaining the position of the motor. This design offers high signal sensitivity and may eliminate missing pulse problems during regenerative braking mode. However, this approach requires the use of a special H-bridge to separate the GND between the FWD and the transistor switches. The system cannot use MOSFET-based H-bridges, otherwise it will miss pulses during the braking mode. Furthermore, since the main motor current has to go through the sensing inductor, excessive voltage drops or power losses will be present if a small inductor is used. The sensed signal may vary among production motors of the same design and over the life span of the same motor due to the use of current pulses associated with delayed commutation.
It is therefore desirable to design a DC motor position detection and control system that will eliminate or minimize the drawbacks associated with the above-mentioned prior art systems. Preferably such a desirable system would not require special H-bridge, or motor, or additional power supplies, would not add additional voltage drops and power losses from the pulse sensing circuit, would have a high useful signal to sensed signal ratio, would not have pulse gaining problem due to brush bounces, would not have pulse missing problems during startup, regenerative braking, or stop modes of operations, would have consistent captured signals for high volume produced motors or over the life span of the same motor, and is independent of the EMI suppression designs.
In view of the above, it is apparent that there exists a need for an improved position detection and control system for a DC electric motor.